Bridge circuits are often used in conjunction with measurement devices to provide highly accurate measurement of variable conditions. The measurement devices used in a bridge circuit are of the type having an impedance which varies in proportion to changes in the condition being measured, e.g., a strain gauge, thermistor, etc. A measuring device is located in at least one leg of the bridge, and an output signal from the bridge is representative of a change in the impedance of the measuring device. Highly accurate measurements are possible because a null condition is used to compare ratios of impedances in the bridge legs.
FIG. 1 is a block diagram of the present method of setting up and calibrating a strain gauge bridge measurement circuit. A strain gauge bridge circuit is used to measure the strain in a component that a strain gauge is mounted to. A strain gauge employs either a conductor or semiconductor, of small cross-sectional area, suitable for mounting to the measured surface so that it elongates or contracts with the surface and changes its resistance accordingly. The bridge is calibrated using a balance potentiometer 10. The balance potentiometer is connected across the bridge and manually adjusted for a predetermined output from an amplifier 12 connected to the output of the bridge. The gain of the amplifier is determined by shunting one leg of the bridge with a precision resistor 15 which is equivalent to a known strain.
A problem associated with balancing a bridge measurement circuit using this method is that it is very time consuming to manually adjust the balance potentiometer to achieve the desired output from the amplifier. Additionally, a precision resistor must be selected for shunting which provides a calibration within the expected range of the output. Numerous shunting resistors must be used to check for a linear response across the entire output range.